Transition-metal nanoclusters: kinetic, mechanistic and arene hydrogenation catalysis studies

Abstract

Following a review of the appropriate literature, the research presented herein includes: (1) the development and testing of an indirect kinetic method for following transition-metal nanocluster formation under hydrogen: (2) the use of polyoxoanion-stabilized Rh(0) nanoclusters for monocyclic arene hydrogenation catalysis: and (3) a demonstration that precipitated ruthenium metal is the true catalyst in the benzene hydrogenation system based on the monometallic precatalyst, Ru(II)(ɳ6-C6-Me6)(OAc)2.

A few years ago a new kinetic method for following transition-metal nanocluster formation was developed in which the resultant nanocluster's catalytic activity was used as a reporter reaction. Herein this new kinetic method is tested and developed further: (i) by following nanocluster formation directly via H2 uptake: (ii) by following nanocluster size vs time via TEM: (iii) by numerical integration simulations of the nanocluster formation reaction: and (iv) by showing that the new kinetic method can be used for a variety of metals and catalytic reactions. In the course of these studies it was discovered that heterolytic hydrogen activation is important for the formation of nanoclusters from higher valent metals.

Well-characterized polyoxoanion- and tetrabutylammonium-stabilized Rh(0) nanoclusters are synthesized by the reduction of [Bu4N]5Na3[(1.5-COD)Rh P2W15Nb3062] with H2 in propylene carbonate solvent. Propylene carbonate solutions of the Rh(0) nanoclusters catalyze the hydrogenation of anisole (methoxybenzene) under mild conditions (22-78 °C. 30-40 psig H2). Proton donors such as water or HBF.4 Et2O affect both nanocluster formation and nanocluster arene hydrogenation catalysis. These Rh(0) nanoclusters are 10-fold more active than a commercially available 5% Rh/AI2O3 catalyst of the same average metal-particle size. The Rh(0) nanoclusters are capable of ≥2600 catalytic turnovers, and also display an unusual selectivity for the partial hydrogenation of anisole to 1-methoxycyclohexene.

Finally, a literature arene hydrogenation system based on the precatalvst Ru(II)(ɳ6-C6-Me6)(OAc)2 is re-investigated. A previously developed, four-step, mechanistic approach is used to answer the question '‘is it homogeneous or heterogeneous catalysis?" The data presented herein provide compelling kinetic (and other) evidence that the true catalyst in this system is not a homogeneous metal complex or a soluble colloid: rather, the data are consistent with bulk ruthenium metal as the true catalyst.